The invention relates to the area of semiconductor technology and concerns a microelectronic structure with a base substrate, a silicon-containing layer and an oxygen-barrier layer.
In order to achieve a further increase in the storage capacity of semiconductor memories, efforts are being made to use high-∈ dielectrics (∈ greater than 20) or ferroelectric dielectrics. During their deposition and conditioning, the preferred high-∈ and ferroelectric dielectric materials require temperatures of up to 800xc2x0 C. and atmospheres containing oxygen. Under such conditions, however, prior art materials used for electrodes can be expected to undergo rapid oxidation. Therefore, the use of oxidation resistant electrode materials was also suggested. Platinum, for example, is a prominent example. When using platinum, however, the problem arises that direct contact of platinum with silicon at the high process temperatures leads to the interfering formation of platinum silicide. In addition, oxygen can diffuse through platinum relatively easily and oxidize the silicon located beneath it. For these reasons a barrier is necessary between the platinum electrode and a contact hole filled with polysilicon that connects the electrode with a selection transistor.
The barriers must meet, in particular, the following requirements. On the one hand they must prevent silicon from diffusing from the contact hole to the platinum electrode and, on the other, they must inhibit diffusion of oxygen from the platinum to the contact hole in order to exclude the electrically insulating oxidation of silicon. Moreover, the barriers themselves must be stable under the process conditions.
A possible design for a microelectronic structure as mentioned at the beginningxe2x80x94in the form of an electrode barrier systemxe2x80x94is described, for example, in U.S. Pat. No. 5,581,439. There, a titanium nitride layer preventing the diffusion of silicon is buried in a silicon nitride layer, which protects the titanium nitride layer from oxidation, at least laterally. The silicon nitride collar supports a palladium base body with a platinum coating, which together form the electrodes. At the same time the titanium nitride layer should be protected from oxidation at least through the palladium.
In contrast, the design of a further electrode barrier system with other materials is described in the publication by J. Kudo et al., xe2x80x9cA High Stability Electrode Technology for Stacked SrBi2Ta2O9 Capacitors Applicable to Advanced Ferroelectric Memoryxe2x80x9d, IEDM 1997, pp. 609 to 612. In the design published there, a barrier of tantalum silicon nitride covered by a layer of pure iridium and a layer of iridium dioxide is preferred. The tantalum silicon nitride barrier prevents the diffusion of silicon but must itself be protected from oxidation. This task is performed by the iridium oxide layer and the pure iridium layer. It has been found, however, that at high temperatures, especially at 800xc2x0 C., the pure iridium layer and the tantalum silicon nitride barrier together form iridium silicide, which has poor electrical conductivity.
The same problems also occur with the design favored by Saenger et al., xe2x80x9cBuried, self-aligned barrier layer structures for perovskite-based memory devices including Pt or Ir bottom electrodes on silicon-contributing substratesxe2x80x9d, J. Appl. Phys. 83(2), 1998, pp. 802-813. This publication reports that an interfering iridium silicide is formed from pure iridium and polysilicon during a recovery step in an atmosphere of nitrogen. This siliconization should therefore be prevented by a preceding recovery step in an atmosphere containing oxygen through complete oxidation of the iridium. Unfortunately this recovery step can be controlled only with difficulty, especially in respect of in-depth oxidation of the iridium, so that if the iridium layer is of uneven thickness oxidation of the polysilicon can also occur, resulting in an interruption of the electrical contact between the polysilicon and the iridium.
The use of a deposited pure iridium layer with subsequent oxygen treatment is also reported in the publication by Jeon et al., xe2x80x9cThermal stability of Ir/polycrystalline-Si structures for bottom electrode of integrated ferroelectric capacitorsxe2x80x9d, Appl. Phys. Lett. 71(4), 1997, pp. 467-469. On the other hand, the use of iridium dioxide as barrier is described in Cho et al., xe2x80x9cPreparation and Characterization of Iridium Oxide Thin Films Grown by DC Reactive Sputteringxe2x80x9d, Jpn. J. Appl. Phys. 36, 1997, pp. 1722-1727. Furthermore, the use of a multi-layer system of platinum, ruthenium and rhenium is known from Onishi et al., xe2x80x9cA New High Temperature Electrode-Barrier Technology On High Density Ferroelectric Capacitor Structurexe2x80x9d, IEDM 96, pp. 699-702; Bhatt et al., xe2x80x9cNovel high temperature multilayer electrode-barrier structure for high-density ferroelectric memoriesxe2x80x9d, Appl. Phys. Lett. 71(5), 1997, pp. 719-721; Onishi et al., xe2x80x9cHigh Temperature Barrier Electrode Technology for High Density Ferroelectric Memories with Stacked Capacitor Structurexe2x80x9d, Electrochem. Soc. 145, 1998, pp. 2563-2568; Aoyama et al., xe2x80x9cInterfacial layers between Si and Ru Films Deposited by Sputtering in Ar/O2 Mixture Ambientxe2x80x9d, Jpn. J. Appl. Phys. 37, 1998, pp. L242-L244.
A further barrier approach is proposed in U.S. Pat. No. 5,852,307, which describes the use of a slightly oxidized ruthenium layer and a ruthenium dioxide layer.
However, all of the barrier layers of prior art are accompanied by the risk that they are no longer sufficiently stable at the high process temperatures required, especially during a necessary temperature step for conditioning the high-∈ materials and/or the ferroelectric materials.
It is accordingly an object of the invention to provide a microelectronic structure and a method for fabricating the structure which overcomes the above-mentioned disadvantageous of the prior art apparatus and methods of this general type, and in particular, to provide a microelectronic structure which is sufficiently stable at temperatures up to 800xc2x0 C.
With the foregoing and other objects in view there is provided, in accordance with the invention a microelectronic structure, that includes: a base substrate; a silicon-containing layer; an oxygen-barrier layer; and an oxygen-containing iridium layer located between the silicon-containing layer and the oxygen-barrier layer. The oxygen-containing iridium layer is a sputtered layer that is fabricated by sputtering in an atmosphere containing oxygen at a temperature of at least 250xc2x0 C. with a volume percent of oxygen in the atmosphere being between 2.5% and 15%.
The oxygen-containing iridium layer in the microelectronic structure prevents diffusion of silicon out of the silicon-containing layer and into the oxygen-barrier layer and any further layers arranged beyond it. To this end the oxygen-containing iridium layer contains a specific proportion of oxygen, which prevents the formation of iridium silicide and thereby the further diffusion of silicon. In addition, the boundary layer between the oxygen-containing iridium layer and the silicon-containing layer also remains essentially free of iridium silicide at temperatures up to at least 800xc2x0 C. This can be proved, for example through resistance measurements of the oxygen-containing iridium layer. The absence of iridium silicide is indicated, for example, by a very low resistivity of the oxygen-containing iridium layer of less than 100 xcexcohm * cm, preferably even less than 30 xcexcohm * cm. In the presence of iridium silicide, which has a very high resistivity of approx. 6 ohm * cm, the resistivity of the structure formed from the silicon-containing layer and the oxygen-containing iridium layer would be considerably greater than 100 xcexcohm * cm. The low electrical resistance of the microelectronic structure is of considerable advantage, especially in maximally integrated semiconductor componentsxe2x80x94especially in semiconductor memories with structural sizes of 0.25 xcexcm and below.
In addition, as a result of the oxygen-containing iridium layer, contact between the silicon-containing layer and the oxygen-barrier layer is avoided as far as possible in order to prevent a possible reduction of the oxygen-barrier layer by the silicon-containing layer and the associated oxidation of the silicon-containing layer.
An oxygen-containing iridium layer with the above characteristics can be fabricated, for example, by means of a sputtering method in an oxygen-containing atmosphere with a low oxygen concentration, whereby the volume percent of oxygen in the atmosphere lies between 2.5% and 15%. Through the limited volume percent of oxygen in the atmosphere, oxygen is also incorporated into the iridium layer to only a specific extent so that it is also possible to speak of an anoxidized iridium layer. The volume percent of oxygen in the atmosphere is preferably approximately 5%.
Experiments have shown that oxygen-containing iridium layers fabricated at a volume percent of approximately 2.5% oxygen still resist siliconization to the maximum possible extent, whereas oxygen-containing iridium layers fabricated in an atmosphere containing less than 2.5% oxygen already have a considerable tendency towards siliconization. On the other hand, an oxygen-containing iridium layer deposited at an oxygen volume concentration of maximally 15% does not yet lead to an interfering oxidation of the silicon-containing layer beneath the oxygen-containing iridium layer.
In accordance with an added feature of the invention, the oxygen-containing iridium layer is deposited at a temperature of at least 250xc2x0 C. in order to improve the adhesion of the oxygen-containing iridium layer. This results, in particular, in an improved adhesion between the oxygen-containing iridium layer and the silicon-containing insulating layers which, for example, consist of silicon nitride and silicon oxide. Since the base substrate itself can consist of silicon oxide or silicon nitride, the deposition of the oxygen-containing iridium layer at increased temperatures also achieves a good adhesion of the oxygen-containing iridium layer to the base substrate. The deposition temperature should principally be chosen high enough to ensure adequate adhesion to the base substrate, whereby this enables an adhesive strength of at least 100 kg/cm2 to be achieved.
A further advantage of depositing the oxygen-containing iridium layer at a temperature of at least 250xc2x0 C. consists therein that a further conditioning step to improve the adhesion of the oxygen-containing iridium layer is not necessary. As long as the deposition temperature chosen is not too high, for example between 250xc2x0 C. and 400xc2x0 C., already created structures are subjected to hardly any thermal stress.
In accordance with an additional feature of the invention, the oxygen barrier consists of a conductive metal oxide. Iridium dioxide and ruthenium dioxide have especially proved themselves to be a suitable metal oxide. The use of these metal oxides also ensures good adhesion of the oxygen-barrier layer to the oxygen-containing iridium layer.
In accordance with another feature of the invention, the silicon-containing layer, which is located as a rule beneath the oxygen-containing iridium layer, preferably consists of polysilicon, of a metal silicide, or of a stack of layers which includes at least one polysilicon layer and one metal silicide layer located between the polysilicon layer and the oxygen-containing iridium layer. Preferably the metal silicide consist of at least one silicide from the group yttrium silicide, titanium silicide, zirconium silicide, hafnium silicide, vanadium silicide, niobium silicide, tantalum silicide, chromium silicide, molybdenum silicide, tungsten silicide, iron silicide, cobalt silicide, nickel silicide, palladium silicide, platinum silicide, and copper silicide. The metal and the silicon can be present in different stoichiometric relationships. The metal silicides used can also be of tertiary structure and comply with the general formula MSiN where M stands for a metal and N stands for nitrogen.
In accordance with a further feature of the invention, it has been found to be advantageous to bury at least the silicon-containing layer in the base substrate and to cover it completely with the oxygen-containing iridium layer. In this way the silicon-containing layer is protected by the base substrate from attack by oxygen, at least laterally, and the silicon located in the silicon-containing layer is prevented from diffusing through the oxygen-containing iridium layer. In this construction the silicon-containing layer can, for example, be present in the form of a contact hole filled with polysilicon, that can optionally be bounded by a metal silicide layer in the direction of the oxygen-containing iridium layer.
It is also advantageous, however, to bury the silicon-containing layer and the oxygen-containing iridium layer together in the base substrate and to cover them completely with the oxygen-barrier layer. In this structure, any problems with adhesion of the oxygen-containing iridium layer to the base substrate are avoided in that the oxygen-containing iridium layer is in contact with the base substrate only on its lateral bounding surface (border). In contrast, the adhesion of the oxygen-containing iridium layer to a conductive silicon-containing layer, especially to polysilicon or a metal silicide, is generally sufficiently strong.
In accordance with a further added feature of the invention, the oxygen-containing iridium layer preferably has a thickness of approximately 100 nm, advantageously even of approximately 20 to 50 nm. Efforts are made to form the oxygen-containing iridium layer as space-saving and as thin as possible. The barrier layers contained in the microelectronic structure (oxygen-barrier layer, oxygen-containing iridium layer) are advantageously covered by a metal-containing electrode layer. The oxygen-barrier layer in particular should be coated as completely as possible by this layer. The metal-containing electrode layer preferably consist of a metal (e.g. platinum, ruthenium, iridium, palladium, rhodium, rhenium, osmium) or a conductive metal oxide (MOx, e.g. ruthenium oxide, osmium oxide, rhodium oxide, iridium oxide, rhenium oxide or conductive perovskites, e.g. SrRuO3 or (La,Sr)CoO3). Platinum is especially preferred as the metal. On the metal-containing electrode layer there is a dielectric metal-oxide-containing layer which, especially in the case of a semiconductor memory, constitutes the high-∈ dielectric or the ferroelectric capacitor dielectric. For the dielectric metal-oxide-containing layer, metal oxides are especially used of the general formula ABOx or DOx, where A stands especially for at least one metal from the group strontium (Sr), bismuth (Bi), niobium (Nb), lead (Pb), zirconium (Zr), lanthanum (La), lithium (Li), potassium (K), calcium (Ca) and barium (Ba), B stands especially for at least one metal from the group titanium (Ti), niobium (Nb), ruthenium (Ru), magnesium (Mg), manganese (Mn), zirconium (Zr) or tantalum (Ta), D stands for titanium (Ti) or tantalum (Ta) and O stands for oxygen. X can be between 2 and 12. Depending on their composition, these metal oxides can have dielectric or ferroelectric properties, whereby these properties may not be detectable until after a high-temperature step for crystallization of the metal oxides. Under certain circumstances these materials exist in a polycrystalline form, whereby perovskite-like crystal structures, mixed crystals or super-lattices can frequently be observed. In principal, all perovskite-like metal oxides of the general form ABOx are suitable for forming the dielectric metal-oxide-containing layer. Dielectric materials with high ∈ (∈ greater than 20) and materials with ferroelectric properties are, for example, barium strontium titanate (BST, Ba1xe2x88x92xSrxTiO3), niobium-doped strontium bismuth tantalate (SBTN, SrxBiy(TazNb1xe2x88x92z)O3), strontium titanate (STO, SrTiO3), strontium bismuth tantalate (SBT, SrxBiyTa2O9), bismuth titanate (BTO, Bi4Ti3O12), lead zirconate titanate (PZT, Pb(ZrxTi1xe2x88x92x)O3), strontium niobate (SNO, Sr2Nb2O7), potassium titanate niobate (KTN) and lead lanthanum titanate (PLTO, (Pb,La)TiO3). In addition, tantalum oxide (Ta2O5) is also used as high-∈ dielectric. In the following, the term xe2x80x9cdielectricxe2x80x9d is used to mean a dielectric, paraelectric or ferroelectric layer, so that the dielectric metal-oxide-containing layer can exhibit dielectric, paraelectric or ferroelectric properties.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of using the microelectronic structure in a semiconductor memory device. The microelectronic structure is preferably used in a semiconductor memory device which has at least a first and a second electrode and between them a metal-oxide-containing layer, which together form a storage capacitor. The first electrode of this semiconductor memory device thereby includes at least the oxygen-containing iridium layer and the oxygen-barrier layer so that in addition to an optional noble metal layer, the first electrode also contains the necessary diffusion barriers.
In a preferred microelectronic structure, the base substrate consists in particular of silicon oxide, silicon nitride or of a layer combination of these materials. The base substrate is interspersed by at least one contact hole filled with polysilicon or a different conductive material. Possibly the filled contact hole is closed off level with the surface of the base substrate with a metal silicide layer arranged in the contact hole. Finally, the oxygen-containing iridium layer sits on the surface of the base substrate, covers the contact hole completely, extends laterally beyond it and is in direct contact with the base substrate where it does so.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for fabricating a microelectronic structure, that includes:
providing a microelectronic structure having a base substrate;
and depositing an oxygen-containing iridium layer on the base substrate by sputtering in an atmosphere containing oxygen at a temperature of at least 250xc2x0 C., whereby a volume percent of oxygen in the atmosphere is between 2.5% and 15%.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a capacitor electrodes arrangement, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.